Highly reliable semiconductor devices require metallic contamination to be very low. Typically, semiconductor substrates are processed in a furnace tube or a rapid thermal processor (RTP) chamber having a single wall made of quartz. Most metallic contaminants migrate very quickly through quartz. Therefore, any metallic contamination present at the outer surface of the single-wall quartz chamber is likely to migrate through the wall and may reach the substrate, particularly at high processing temperatures (900 degrees Celsius and higher).
A double-walled quartz tube having hydrogen chloride (HCl) gas between its walls has been used in an attempt to reduce metallic contamination. Still, HCl gas has problems. HCl is corrosive and is irritating to humans.
Polycrystalline silicon (polysilicon) and silicon carbide liners have been used inside or outside furnace tubes. When a liner is used within a furnace tube, particles are generated as the liner is inserted into the tube. Particles generated by the insertion may end up on substrates that are processed within the tube. Generally, a higher number of particles on a substrate translates to lower yield. The liner may also be placed outside of the tube. Polysilicon and silicon carbide are not insulators. If the liner contacts the furnace element or other wiring, an electrical short may occur, and the furnace may be damaged. Even worse, if a person is working with the furnace and the liner contacts the furnace element, the person may be electrocuted if the furnace has not be properly disabled.